In hydrocarbon exploration and production there is a need to determine the approximate composition of oil samples in order to determine the origin and properties of the oil. The analysis of oil samples to determine the approximate composition thereof and more particularly, to obtain a pattern that reflects the composition of a sample and that can be recognised, is known in the art as fingerprinting.
There are many known methods of fingerprinting. Most of these methods use gas chromatography (GC) to separate out individual components of a complex hydrocarbon mixture. Some methods use the combination of gas chromatography and mass spectroscopy (GC-MS) to detect spectra characteristic of individual components of the complex hydrocarbon mixture.
Most fingerprinting techniques known in the art are based on the identification and quantification of a limited number of selected compounds which act as marker molecules. One such method is described in U.S. Pat. No. 5,602,755A to Ashe et al. This document discloses a method for predicting the properties of a complex hydrocarbon mixture which comprises selecting one or more known chemical, perceptual, physical or performance properties of the complex mixture and creating a training set from reference samples which contain characteristic molecular species present in the mixture. The training set is produced by GC-MS analysis of the reference samples and is then used to determine a predicted value of the property of an unknown mixture from a GC-MS analysis thereof.
There is a need in the art however for more complete characterisation of very complex hydrocarbon containing mixtures. A method of more complete characterisation has been developed and successfully used on extracts of diesel exhaust particles. This method uses full scan GC-MS analysis of the sample followed by curve resolution of the results of the analysis to obtain peaks and spectra representing individual compounds in the sample. This method is described in Eide et al. 2001. Environ. Sci. Technol. 35, 2314-2318. A problem with this method is that the heavier parts of oils (those with a boiling point of 400° C. to 450° C.) are difficult to analyse. In addition, the use of curve resolution becomes very complicated with crude oils, which are extremely complex mixtures. Still further, the use of GC-MS analysis is time consuming.
An alternative known method uses high resolution GC-MS analysis to give a much higher resolution of spectra obtained than standard GC-MS (about 10 times as many peaks are obtained using the high resolution method). However, in high resolution GC-MS analysis of a number of oil samples, a very long time is needed to carry out each analysis. Using this method, the data obtained may be too complex to allow chromatograms obtained to be resolved into individual peaks and thus interpret the pattern obtained. Further, the method does not work on large (heavy) molecules.